MySQL JOIN table based on MAX(date) in main table, and MAX(id) in joined table with LIMIT

Question

如果标题没有任何意义，这就是我需要做的事情 shell.. 我需要 select 最近的 X 条记录 "by date"在主要 table 中，然后通过 select 合并最近的记录 "by id" 加入属于这些记录的数据 table..

这是一些示例输出..

table: lead_unique（此 table 中只有唯一的 ssn）

+-----------+--------------+
| ssn       | created_date |
+-----------+--------------+
| 111111111 | 2015-03-01   |
| 999999999 | 2015-03-03   |
| 555555555 | 2015-02-08   |
+-----------+--------------+

table: lead_data

+----+-----------+-------+----------------+-------------+-------+-------+
| id | ssn       | name  | address        | city        | state | zip   |
+----+-----------+-------+----------------+-------------+-------+-------+
|  1 | 111111111 | Bob1  | 1234 Test Ln   | Mound       | CA    | 55555 |
|  2 | 111111111 | Bob2  | 1234 Test Ln   | Mound       | CA    | 55555 |
|  3 | 999999999 | Jane1 | 5432 Lola Blvd | Patton      | NJ    | 33333 |
|  4 | 999999999 | Jane2 | 5432 Lola Blvd | Patton      | NJ    | 33333 |
|  5 | 555555555 | Jack1 | 832 92nd Ave N | Bright View | AL    | 88888 |
|  6 | 999999999 | Jane3 | 5432 Lola Blvd | Patton      | NJ    | 33333 |
+----+-----------+-------+----------------+-------------+-------+-------+

期望的输出（可以是 asc/desc 日期列，无关紧要）

+--------------+-----------+-------+
| created_date | ssn       | name  |
+--------------+-----------+-------+
| 2015-03-03   | 999999999 | Jane3 |
| 2015-03-01   | 111111111 | Bob2  |
| 2015-02-08   | 555555555 | Jack1 |
+--------------+-----------+-------+

期望输出（限制 2）

+--------------+-----------+-------+
| created_date | ssn       | name  |
+--------------+-----------+-------+
| 2015-03-03   | 999999999 | Jane3 |
| 2015-03-01   | 111111111 | Bob2  |
+--------------+-----------+-------+

查询可能类似于以下内容，但我也可能会遇到麻烦，因为我在这里寻求帮助但运气不好..

SELECT   
    lead_unique.created_date, 
    lead_unique.ssn,
    lead_data.name
FROM      
    lead_unique
JOIN      
    (
        SELECT    
            ...
        FROM      
            lead_data
        ...
    ) lead_data 
        ...
...
LIMIT 2

我之前只用过一次堆栈溢出，所以如果我还有什么可以补充的，请告诉我！谢谢！！

Answer 1

我倾向于对数据块使用相关子查询——你的问题只提到了一列：

select u.created_date, u.ssn,
       (select d.name
        from lead_data d
        where d.ssn = u.ssn
        order by d.id desc
        limit 1
       ) as name
from lead_unique u
order by u.created_date desc
limit 2;

实际上，出于性能原因，我会将唯一组件放在子查询中：

select u.created_date, u.ssn,
       (select d.name
        from lead_data d
        where d.ssn = u.ssn
        order by d.id desc
        limit 1
       ) as name
from (select u.*
      from lead_unique u
      order by u.created_date desc
      limit 2
     ) u;

编辑：

即使有多个列，最高效的方法可能仍然是使用子查询：

select created_date, ssn, d.*
from (select u.created_date, u.ssn,
             (select d.id
              from lead_data d
              where d.ssn = u.ssn
              order by d.id desc
              limit 1
             ) as id
      from (select u.*
            from lead_unique u
            order by u.created_date desc
            limit 2
           ) u
     ) u join
     lead_data d
     on u.id = d.id;

顺便说一下，如果性能是一个问题，您需要以下索引：lead_unique(created_date) 和 lead_data(id)。这两个索引应该很快。

MySQL JOIN table based on MAX(date) in main table, and MAX(id) in joined table with LIMIT

MySQL JOIN table based on MAX(date) in main table, and MAX(id) in joined table with LIMIT

mysql

join

limit